Use of Short Radius Horizontal Recompletions to Recover Un-Swept Oil in a Maturing Giant Field

Abstract

This study evaluates the performance of short radius horizontal (SRH) wells in a giant field in Saudi Arabia. The objective of these SRH recompletions was to reactivate the wells that were dead due to excessive water production from an advancing flood front. The paper discusses factors contributing to success or failure in meeting the objective and compares the performance with the vertical dead wells that were plugged-back with a mechanical plug to reduce water production and to enliven them. This study concludes that sidetracking dead wells and re-completing them in a relatively thin, upper section of the reservoir is an economically viable strategy to target the remaining oil. The productive life of short radius horizontal wells was found to be longer than that of vertical wells that were plugged back with a mechanical barrier in the wellbore. Introduction Vertical oil wells in this field cease to flow at water cuts ranging from 70–80% due to high operating pressure at the Gas Oil Separation Plants (GOSPs). An unswept oil column of approximately 30 ft. is left at the top of the reservoir. This unswept oil, an asset too valuable to leave behind, is distributed across Zone-1 (20 ft. thick) and the top 10 ft. of Zone-2. Horizontal drilling technology has made it possible to plug back a dead well and target a 1,000 ft., short radius horizontal drainhole in the 30 ft. unswept oil column. Most wells in the field are drilled vertically and they die at water cuts ranging from 70 – 80% due to high back pressure of 450 psig at each of the four GOSP's. Zone-1 typically has two lobes that are separated by a thin streak of anhydrite or very low porosity (Fig. 1). In addition, Zone-1 is separated from Zone-2 by a low porosity - low permeability isolation section, sometimes associated with dolomitic lithology. This study reviews 13 wells that were sidetracked and re-completed as short radius horizontal producers in Zone-1 of Arab-D reservoir. The objective of the wells was to target unswept oil in Zone-1 and the top of Zone-2 that cannot be produced with vertical wells due to high water production from lower zones and back pressure from GOSPs which are operating at 450 psig. Factors contributing to the success of wells were found to be:Absence of natural fractures between zones that can cause premature water breakthrough.Existence of good natural barrier between Zone-1 and Zone-2.An optimally placed lateral open hole section in the lower lobe of Zone-1.Adequate vertical standoff between the horizontal open hole section and top of Zone-2. The study also evaluates well performance with that of vertical plugged-back wells. General Field Description The field is a northeast-southwest trending, elongated anticline with a south dome. It is a carbonate reservoir that contains Arabian Extra Light crude. There are four GOSPs in the field operating at 450 psig. pressure and all of them are equipped with wet crude handling facilities. The field has a modest natural water drive that has been augmented with a peripheral water injection program since the mid 50s. Oil production is supported by power water injection (PWI) around the northern end of the reservoir and on the southeast flank and gravity water injection (GWI) around the remaining periphery. Porosity profile of the reservoir is shown in Fig. 1.